Multiple LFMC Issues, Back-Fed OCPDs and More

If you have a National Electrical Code (NEC)-related problem, are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to letthecodedecide@earthlink.net.

LFMC maximum length

Is there a maximum length for liquidtight flexible -metal conduit (LFMC)? I know, with more than 6 feet, you have to run a ground wire, and it has to be secured every 4 feet unless fished in walls. But the other day, I was told that the NEC does not permit using LFMC in lengths of more than 6 feet. I checked the 2005 NEC Article 350, and related articles 250 and 300, but could not find any wording to that effect. Did it change in the 2008 Code, or did I miss something in the 2005 NEC?

Liquidtight flexible metal conduit (LFMC) may be installed in unlimited lengths, provided it meets the other requirements of Article 350 and a separate equipment-grounding conductor is installed with the circuit conductors. There are some 6-foot rules relating to LFMC. Section 250.118(6)(d) does not permit LFMC to exceed 6 feet in the same ground return path. Section 230.43(15) limits LFMC to 6 feet between raceways or between raceway and service equipment. These references from the 2008 NEC are the same in the 2005 NEC.

Fastener requirements for back-fed OCPDs

Can you give any insight to the 408.36(D) requirement for an additional fastener to be used when back-feeding plug-in-type overcurrent-protection devices (OCPDs)? I do not understand what difference feeding a load through a plug-in breaker versus back-feeding through the same breaker can have on the integrity of the plug-in connection. Why does that require an additional fastener?

I believe you are right about the integrity of the plug-in connection being relatively the same, but when back-feeding to supply a panel, the consequences of a failure due to a poor connection are considerably more problematic. Losing a circuit as opposed to losing a complete panel warrants the extra precaution of an additional fastener. So a little more security was added.

LFMC vs. FMC confusion

I work as an industrial electrician, and we use flexible metallic conduit “Sealtight” on many applications where vibration is a nuisance. You wrote that a new Code rule of 2008 NEC to 348.12(1) now specifically states that FMC shall not be used in wet locations. I was under the impression that this was the purpose of this type of conduit, to provide a flexible installation that could be used in wet locations and also provide relief from stress of vibration. Is this ruling for commercial installations, or is it for all locations?

You’re mixing flexible metal conduit (FMC) up with LFMC. The requirements for FMC are found in Article 348, and the requirements for LFMC are found in Article 350. Codes prior to the 2008 edition permitted FMC to be used outdoors in wet locations if the conductors were suitable for wet locations. It was determined that, in many installations, water would run through the FMC and get into the equipment. A proposal was made to the Code-Making Panel to prohibit using FMC in wet locations. This proposal was accepted, and FMC now cannot be used in wet locations (348.12(1)).

Romex run in PVC conduit

I was wondering if it is legal to run 12/3 Romex in PVC to feed an above-ground pool if you don’t use the bare conductor as a ground. Does the Code say that you cannot run Romex in PVC?

Nonmetallic-sheathed cable (Type NM) can be enclosed in Schedule 80 PVC for protection from physical damage. Type NM cable is not permitted to be run in damp or wet locations (334.12(B)(4)). The interior of raceways run outdoors is considered to be a wet location, and Type NM cable cannot be used.

Conduit fill

When calculating conduit fill, do you count the ground wire? I have 13 No. 10 THHN wires in a 3/4-inch rigid galvanized conduit and told the owner that is a Code violation. I was told the ground wire does not get included.

According to Table C1 in Chapter 9, you are permitted to install 10 10 AWG THHN conductors in a -inch conduit. Apparently, you have installed 13 conductors based on the assumption that ground conductors are not required to be counted. I don’t know whether you mean grounded (neutral) conductors or grounding (equipment) conductors, but either way, they must be counted. Note 3 to Table 1 requires equipment-grounding conductors to be counted when calculating conduit fill.

Raceway seals

NEC article 300.5(G) requires conduits or raceways to be sealed or plugged when moisture may contact energized parts. Does this mean that all buried raceways or conduit must be sealed at the box, enclosure, etc., above ground? How do you determine when moisture becomes a problem and needs to be sealed? Also, will duct seal suffice?

No, this does not mean that all buried raceways must be sealed. However, if a buried raceway terminates in equipment that is installed below grade level, such as underground service-entrance conductors entering a service panel, sealing would be required. NEC 300.7, relating to raceways exposed to different temperatures, shows instances where raceways are subjected to different temperatures and condensation becomes a problem, which would require sealing. Sealing fittings, such as those required for hazardous locations, are not necessary. Duct seal should be OK, but check with the authority having jurisdiction (AHJ).

Plenum ceilings

If LFMC is not allowed in a plenum ceiling anymore, what can be used in its place?

If you are following the NEC, the wiring methods permitted are shown in Article 300.22(B). FMC is permitted in lengths not exceeding 4 feet to connect physically adjustable equipment and devices permitted to be in plenum chambers. The exception to 300.22 that permitted LFMC was deleted in the 2002 Code because of the toxicity of the sheath in case of a fire. I believe you are using the Chicago Code, which is basically the 2002 National Electrical Code.

Running boards

Would you please explain the need to use running boards as noted in Section 334-15(A) and (C). If you secure the cable at each floor joist, you eliminate the sag problem and get the same coverage as boring through the joist. I know this has been in the Code for a long time, but does that make it right?

NEC 334.30 requires that Type NM cable be supported and secured every 4 feet. Where passing through holes drilled in joists, adequate support is achieved. Fastening to the bottom of joists every 4 feet does not give adequate support. The use of running boards provides adequate backing to support the cable.

GFCI protection for outletsDo 210.8 exceptions allow the use of nonground-fault circuit interrupter outlets for truck block heaters as long as the installation complies with the assured grounding program and has qualified personnel plugging them in? My thought is that the block heaters are not compatible with the sensitivity of GFCI protection and that the need for the block heaters would be negated by nuisance trips. The installation in question is a truck parking area in which refuse hauling trucks are parked overnight. The refuse hauler has an employee who plugs the block heaters in at night, unplugs them in the morning and then starts the trucks.

NEC 210.8(B)(4) Exception No. 2 applies only to industrial establishments. If the AHJ classifies this parking area as part of an industrial establishment, perhaps you could apply the exception, but it’s unlikely. The NEC has similar requirements in 626.24(D) for electrified truck parking spaces. The use of GFCI-protected receptacles in outdoor areas is considered necessary for the safety of personnel and should be followed. I really don’t see a problem of nuisance tripping in this application and an alarm system could be incorporated to detect a tripped device. The NEC requirements are the minimum, (90.1(B)) and the designer can expand on them.

NM cable for outside lights Can I use NM cable to feed the outside lights and receptacles in a dwelling unit?

No, Section 300.9 was added to the 2008 NEC, and it specifically shows that the interiors of raceways and enclosures installed in above-grade wet locations are considered to be wet locations and require conductors suitable for use in wet locations in accordance with 310.8. NEC 34.12 does not permit Type NM cable to be used in a wet or damp location.

Changing a single-pole switchDoes 210.4(B) now state that all three circuits of a three-phase, 4-wire branch circuit would be turned off in order to change a single pole lighting switch on one circuit in an electrically safe way?

A three-phase, 4-wire branch circuit is a multiwire circuit. NEC 210.4(B) requires that each multiwire branch circuit must have a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates. This means that a 3-pole circuit breaker must be used in the panel to feed this circuit.